Instead, a street vendor pointed out the smoking Pathfinder to mounted police officers, leading to Shahzad's capture more than 50 hours later. In the end it wasn't high-tech 21st century surveillance that caught the crook, but good old-fashioned community vigilance. "The successful model for getting the alleged 'bad guy,''' wrote the Boston Globe, "is more Sam Spade than Jack Bauer."

For years, video surveillance has been seen as a potent weapon in the fight against urban crime. The Department of Homeland Security lays out millions of dollars to throw a surveillance net on our cities. Last year, it spent more than $830 million in 64 metropolitan areas as part of its Urban Area Security Initiative – up from $15 million and seven cities for the same program in 2009. This year the total is $662 million across 31 cities.

Yet the question of effectiveness has haunted governments, police officials and academic researchers for decades. It should also haunt taxpayers, because camera surveillance doesn't come cheap. London's 10,000 camera system, for example, has cost more than $320 million to set up and maintain.

The answer, thus far, has been decidedly mixed. Studies in San Francisco and London – two cities on opposite sides of the camera-density spectrum – found little to cheer. San Francisco's 68 cameras placed in high-crime areas failed to reduce assaults, sex offenses and robbery, and merely moved murder down the block, according to a UC-Berkeley report.

A 2009 meta-analysis by researchers from Northeastern University and the University of Cambridge examined 44 previous studies and turned up some positive results. They found surveillance systems to be most effective in parking lots, cutting crime by 51 percent. Cameras in public transport areas – at subway stations, on trains and at bus stops – generally reduced crime by almost one quarter. And camera systems in public settings cut crime by about seven percent.

Britain – with as many as 4 million cameras across the country – accounted for the majority of these reductions.

Now comes a rigorous new study from the Urban Institute, analyzing surveillance systems in Chicago, Baltimore and Washington, D.C. The Department of Justice's Office of Community Oriented Policing Services (COPS), which has invested more than $16 billion to advance community policing at the state and local levels since 1994, sponsored the study, released in September.

The researchers focused on select high crime areas where cameras had recently been installed and studied crime statistics going as far back as 2001 to include before and after data. In Baltimore, crime fell by 25 percent in one area, 10 percent in another and yet stayed the same in a third. In Washington, cameras appeared to have no effect on criminal activity.

In Chicago, the country's most extensive, integrated network, cameras in Humboldt Park correlated to a 12 percent decline in overall crime, including a 33 percent reduction in drug offenses and robberies and a 20 percent drop in violent crime. Meanwhile, a second Chicago area of study, West Garfield Park, saw no crime drop.

The Urban Institute researchers made two important advances over previous studies. First, they reviewed each cities' decision-making process, the set-up of the surveillance system and finally usage. They found that active monitoring by trained personnel had a greater impact than cameras merely left to record video for later use, in the event of a crime in that area. They also found that costs, if not monitored, can spiral out of control.

Second, the researchers devised a system for calculating the social and governmental costs of various crimes, including expenses related to arrest, pre-sentencing, incarceration and cost to victim. “No prior research has sought to explore the degree to which camera use is cost-beneficial—a critical inquiry in light of the economic challenges currently being experienced by jurisdictions across the country,” they wrote in the executive summary.

Under their system, murder costs $1.4 million, aggravated assault $89,000, robbery $120,000 and rape just $62,000. Baltimore saved $1.50 for every dollar spent on crime cameras, according to the report. The crimes prevented in Humboldt Park saved Chicago a whopping $4.30 for every dollar spent on both the Humboldt and West Garfield systems.

In the end, the Urban Institute researchers offer a handy guide for city officials and law enforcement agencies, with tips from the reasonable (“assess your needs and budget before investing,”) to the mundane (“weigh the costs and benefits of using active monitoring”).

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Their best advice is to manage expectations about the impact of crime camera systems: “Footage quality may be adversely impacted by darkness, inclement weather, equipment damage or dirt;” "Images can be grainy, cloudy, or otherwise unclear;" and “cameras may be diverted to another viewable area when an incident occurs and catch little or nothing of the incident itself."

Aaron Doyle, a criminologist at Carleton University who is part of the Surveillance Studies Centre at Queen’s University and co-editor of a book out next month called Eyes Everywhere: The Global Growth of Camera Surveillance, sees this study as consistent with previous work.

"The worst-case scenario is that these positive results in two of the three cities will be over-hyped and lead to the kind of mega-expensive runaway train that CCTV has been in Britain,” says Doyle. “It is possible that a modest well-planned network could be part of a range of measures that would help in some limited contexts, but I think crime is better dealt with in ways that build and involve community."